Give Phame blogs an explicit 404 controller

Summary:
Ref T11076. Ref T9897. Bad links on Phame blogs are currently made worse because we try to prompt you to login on a non-cookie domain.

Instead, just 404 in a vanilla way. Do so cleanly on external domains.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9897, T11076

Differential Revision: https://secure.phabricator.com/D16010